This requirement arises especially for military reconnaissance and for remote reconnoitering in civil areas. So-called opto-mechanical scanners are used among others for this purpose, whose operational mode is generally known (see for instance Hofmann, O.: Image Quality of Active and Passive Scanners. Photogrammetry and Aerial Imaging 51, 1983, Vol. 3, pages 103-117) . One detector or a plurality of detectors, arranged in the image plane of a lens, scan the terrain linewise transversely to the direction of flight. The scanning is accomplished by an optical deflection mechanism arranged upstream of the lens, typically a rotating mirror. One scanning line follows another transversely to the direction of flight because of the forward movement of the carrier, so that a terrain, or object strip is scanned. This is referred to in the claims as a plurality of periodic scanning sweeps of the object.
A fundamental problem of this method, especially during low flight or with large viewing angles, is that the distance from the scanner to the surface of the terrain or the object along a scanning line varies considerably and because of that the object point size acquired by the detector varies also provided that the focal length and the detector remain constant. Thus, overlaps with increasing viewing angle occur, since the terrain strips scanned by a detector increase in width with increasing viewing angle. This affects image resolution.
Basically a constant resolution of the object can be achieved with changing object distance by varying the focal length or size of the detector.
A known solution realized by the Texas Instruments firm (see J. M. Kaltenbach "Special Method for Avoiding Geometric Errors in the Course of IRLS", Carl-Cranz-Society, 47th Year of Publication page 1.08, Nov. 1986) consists in assembling the detector from individual partial detectors and to retrieve the signals of the individual partial pieces corresponding to the distance. The solution here is a special detector whose partial pieces have different shapes and sizes and which permits in each case only the scanning of one line in the direction of scanning in the course of a scanning process.